It is already known to prepare expanded, shaped products by extrusion-cooking which is a short-time, high-solids process in which components are mixed, sheared and subjected to elevated temperatures and pressures, to produce products having a plastic consistency which are shaped by passage through a forming die, whilst in the plastic phase, and then, cut to length. Expansion of size occurs, owing to the release of the pressure upon die exit and normally, to release of gas(es) such as steam. With the prior art, such a procedure results in non-homogeneity of the expansion both within each individual product portion and from one product portion to another product portion. Expanded extrusion-cooked product pieces therefore are known to suffer limitations in their sharpness of features, in their presence of deformities of shape, in their variability of surface characteristics and in their tendency to clump and stick together.
The positions, size and surface activity of minute internal surfaces within the mass of extrusion-cooked products determine which sites are those upon which the gas generation is nucleated. The degree of subsequent expansion, to generate hollows, is variable and dependent upon the strengths of the layers of surrounding plastic mass, which entrap the gas until rupture occurs and the gas escapes from the structure. The problem is that such a procedure results in pieces of an uneven geometry. With uneven geometry, the cosmetic appearance of the shape and its packing fraction are influenced. In addition, an uneven geometry results in products having some thin parts which more easily break and thereby make a considerable contribution to undesirable dust in subsequent handling operations.
Variable pore size and variable wall thickness also influence the rates of hardening of the different parts of the external surfaces of the cut products. This results in a degree of stickiness on parts of the surfaces which causes some cut products to adhere to each other after cutting, which may be permanent or semi-permanent. In the case of permanent adherence, the products are of reject quality. In the case of semi-permanent adherence, the product portions harden at slower rates at the temporary point of attachment, and usually suffer deformity upon detachment.
Variable hardening rates on the surfaces of the product portions also influence the porosity of such surfaces. This porosity variability influences the visual appeal and color of the product, as well as its water absorption or solubility parameters.
With such characteristics, it is necessary to use relatively high energy to prepare such products and suffer low throughput rates. This causes a high degree of wear and tear on the equipment.
Prior art has considered reducing variability in the expansion characteristic by the addition of fat to the product formulation. Although such an approach significantly reduces the total expansion that occurs, it increases fragility, increases energy requirements, reduces production throughput and has dietary implications. Similarly, dry emulsifiers have been added as part of the feedstock to the cooker extruder, but in this form, they fail to significantly influence the degree of non-uniformity in expansion characteristic. U.S. Pat. No. 5,260,078 concerns such a process and furthermore, in this patent, at the extrusion exit, the product is furthermore coated with a vegetable oil and a second surfactant dispersed in the to obtain a desirable level of flavoring.